One of the latest trends in imaging devices is miniaturization. Compact imaging systems, such as miniature cameras, have become ubiquitous with the proliferation of cell phones and other portable handheld devices with cameras integrated therein. While the currently available, compact imaging devices are adequate for low resolution image capture for personal enjoyment, most provide rather low imaging quality or are undesirably long.
An exemplary imaging system 10 is shown in FIG. 1. System 10 may be, for example, a miniature camera and is shown to include a group of optical components 2 (shown here to include two separate refractive elements) and a detector 4. Optical components 2 may be made of an optical material such as PMMA forming four aspheric surfaces, providing a focal length of 2.6 mm and an F# of 2.6 over a 60 degree full field of view. Light rays 5 from an object (not shown) are directed through optical components 2 generally along a Z direction 3, and are imaged onto detector 4. Detector 4 then converts to the image received thereon into a data signal (indicated by a large arrow 7), which is directed to a processor 8. The data signal is processed at a signal processor 18 to result in a final image 9.
Still referring to FIG. 1, optical components 2 of system 10 are located such that a Z-length (defined as the distance from the first surface of the group of optics encountered by an input light ray to the front of the detector, and indicated by a horizontal double-headed arrow) is approximately equal to a length L of detector 4 (indicated by a vertical double-headed arrow). In the exemplary imaging system shown in FIG. 1, detector length L is 4.4 mm, while Z-length is set at 4.6 mm.
Continuing to refer to FIG. 1, system 10 (like numerous other short imaging systems) does not have sufficient degrees of freedom to control the variety of optical and mechanical aberrations that are possibly manifest in the system. That is, since there are so few parts forming the system (e.g., just a few lenses and their holders, small detector, etc.) and the components are so small in compact applications such as a miniature camera, it is difficult to achieve an ideal design or alignment of the different components and/or to adjust any of the components once assembled. As a result, the resulting images do not have high image quality. Further, the potential for introduced aberrations due to misalignment of the physical components (e.g., optical components 2 and detector 4) of system 10 is significant, thereby requiring increased precision during manufacture. This requirement increases the cost of system 10, even though the image quality of the resulting system is relatively poor.
Additionally, in prior art short refractive imaging system 10, the angles of rays at the edge of detector 4 may be shallow. That is, an angle θ of the chief ray (which is the light ray passing through the center of the aperture defined by optical components 2) at the edge of the detector may be up to approximately 30 degrees from the normal of the detector. Since the intensity of light captured at the detector is a function of the angle to the detector, the captured light intensity decreases as the chief ray angle increases. Also, large ray angles may lead to light being captured by the wrong pixel on the detector, thereby causing pixel cross-talk. Therefore, as images formed with practical CMOS, CCD, and IR detectors are degraded when the incident light rays are far from the normal of the detector, large chief ray angles are undesirable. As the Z-length of the system is additionally shortened in an effort to further miniaturize the system, these ray angle problems are exacerbated and increasingly lead to reduced image quality.